Implant for use for adjacently arranged bone plates

ABSTRACT

An implant for use for adjacently arranged bone plates is described, having a first bearing part, a second bearing part and a coupling element for tension-proof coupling of the first bearing part to the second bearing part. The bearing parts are curved concavely and comprise a plurality of ribs, which extend radially outwards from a central portion of the respective bearing part. The upper bearing part comprises a main body and an insert connected thereto by means of at least one connecting element. The connecting element is supported against the coupling element with regard to movement into a release position. The upper bearing part additionally has a deflectable fixing element, which is supported against the upper bearing part in the event of movement of the upper bearing part away from the lower bearing part.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C. §371of International Application No. PCT/EP2007/011024, filed Dec. 14, 2007,published in German, which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an implant for example for fixing a bone flapto a cranial bone or for covering burr holes in the cranial bone. Theimplant has a first bearing part, a second bearing part and a couplingelement for tension-proof coupling of the first bearing part with thesecond bearing part, in such a way that the bone flap and the cranialbone are arranged between the two bearing parts.

BACKGROUND OF THE INVENTION

Prior to removal of a brain tumor from under the cranial bone, thecranial bone has to be opened to provide access to the tumor. To thisend, after the introduction of at least one burr hole into the cranialbone, a bone flap is sawed out of the cranial bone. After surgicalremoval of the tumor, the cranial bone has to be closed again. To thisend, the previously sawed-out bone flap is generally refixed to thecranial bone by means of suitable implants. These implants arefrequently also used to cover the burr holes.

Various implants are known for fixing a bone flap to a cranial bone andfor covering burr holes. For instance, a brochure by Codman & Shurtlefffrom 1965 describes the “Todd Burr Hole Button.” This implant comprisesa first, disc-shaped bearing part with a shank projecting centrally fromthis bearing part. A second, likewise disc-shaped bearing part has acentral opening and is threaded onto the shank in such a way that thelatter extends through the opening.

Like the two bearing parts, the shank is made from a resilient plasticsmaterial and has an external diameter which is somewhat larger than theinternal diameter of the opening in the second bearing part. A tensileforce acting axially on the shank therefore brings about resilientextension of the shank and a reduction in the external shank diameterassociated with the extension. As a consequence of this reduction inexternal diameter, the second bearing part may be threaded onto theshank and moved along the shank towards the first bearing part. If thetensile force acting on the shank is reduced, the external diameter ofthe shank increases again and ensures tension-proof coupling of thesecond bearing part with the shank and thus also with the first bearingpart provided at the shank end.

A number of such implants are used to fix a bone flap to a cranial bone.Generally, a shank acting as coupling element for the two bearing partsis inserted into a gap between the bone flap and the cranial bone insuch a way that the bone flap and the cranial bone are clamped betweenthe two bearing parts.

The implant known from the Codman & Shurtleff brochure has not becomeestablished in practice. One of the reasons for this is the difficultyin handling the implant. For instance, the application of a tensileforce to the shank and the displacement of a bearing part along theshank while the tensile force is maintained has proved extremelyawkward. In addition the implant is not capable of maintaining highclamping forces reliably and in the long term.

JP 05 220 174 A discloses a further implant for fixing a bone flap to acranial bone. The implant comprises a rectangular first bearing partwith a strap projecting there-from and a rectangular second bearingpart. The second bearing part has a central opening and is threaded ontothe strap in such a way that the strap extends through the opening.

The implant strap is provided on one or both sides with teeth. Thebearing part threaded onto the strap has up to two movable claws, thatengage with the teeth provided on the strap. Both the teeth and theclaws have a sawtooth-like profile, that provides less resistance tomovement of the threaded-on bearing part towards the other bearing partthan to movement in the opposite direction. This situation ensurestension-proof coupling of the two bearing parts, in such a way that thebone flap and the cranial bone may be clamped reliably therebetween.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the present invention provides an implant for fixingtwo adjacently arranged bone plates that exhibit improved structuralstability with regard to the tensile and clamping forces arising in thecourse of fixing the implant in place.

According to a first aspect of the present invention, an implant for usefor adjacently arranged bone plates includes a first bearing part, asecond bearing part, having at least one deflectable fixing element, andhaving a coupling element extending through the second bearing part andcooperating with the fixing element for tension-proof coupling of thefirst bearing part with the second bearing part, the coupling elementbeing insertable into a gap between the bone plates in such a way thatthe two bone plates are arranged between the two bearing parts. Acounter support for the at least one fixing element is provided on thesecond bearing part, which counter support is capable of supporting thefixing element in the event of movement of the second bearing part awayfrom the first bearing part.

The fixing element may be supported in that the counter support limitsdeflection of the fixing element towards the first bearing part to apredetermined distance. According to a first variant, this distanceamounts to no more than one or two millimeters, and the distance mayeven be limited to a few tenths of a millimeter. According to a secondvariant, the predetermined distance amounts to roughly zero. In otherwords, the fixing element may already rest against the counter supportin its initial or normal position.

According to another aspect of the present invention, the second bearingpart has at least two fixing elements opposite one another with regardto the coupling element, one counter support being provided for eachfixing element. Thus, in the case of a strap-like coupling element twofixing elements may for example be provided, each fixing elementcooperating with one of the two long sides of the strap. In the case ofa connecting element of round or square cross-section, it is alsopossible to provide three, four or more fixing elements.

According to yet another aspect of the present invention, the at leastone fixing element is deflectable in a direction parallel to alongitudinal axis of the coupling element. In this context the at leastone fixing element may take the form of a tongue or claw. A tongue maybe defined by two or more material incisions.

According to still yet another aspect of the present invention, thesecond bearing part has a main body and an insert attached thereto. Themain body and the insert may consist of identical or indeed of differentmaterials. Plastics and metals in particular are possible materials.

In the case of a multipart second bearing part the at least one fixingelement may be formed in the insert, while the counter support is formedon the main body of the second bearing part. Alternatively, the fixingelement may be provided on the main body and the counter support on theinsert. In other embodiments, there may be a one-piece second bearingpart, such that the fixing element and the counter support may beprovided in one and the same component.

The coupling element may be provided with surface contouring. The atleast one fixing element may cooperate with the surface contouring toachieve tension-proof fixing of the coupling element to the secondbearing part. The surface contouring may comprise a series of teeth ofany desired shape with a symmetrical or asymmetrical profile. Accordingto one variant, the teeth have an asymmetrical (for example asawtooth-like) profile, such that there is less resistance to movementof the second bearing part towards the first bearing part than tomovement in the opposite direction.

According to a further aspect of the present invention, an implant foruse for adjacently arranged bone plates, comprises a first bearing part,a second bearing part with a main body and with an insert attached tothe main body, which insert comprises an opening, and a coupling elementextending through the opening in the insert for tension-proof couplingof the first bearing part with the second bearing part, and the couplingelement being insertable into a gap between the bone plates in such away that the two bone plates are arranged between the two bearing parts.The insert is connected to the main body by means of at least oneconnecting element. The connecting element is supported against thecoupling element with regard to movement into a release position.

The at least one connecting element may be provided on the main body andcooperate with the insert or be provided on the insert and thencooperate with the main body. Alternatively, the at least one connectingelement may be provided on the main body and at least one furtherconnecting element may be provided on the insert.

The insert and the main body may be connected together in different waysby means of the at least one connecting element. In this contextinterlocking and non-interlocking connections may be mentioned by way ofexample. According to one embodiment, the connecting element takes theform of a latching element. The latching element may comprise a snap-inhook, which is supported against the coupling element with regard tounlatching.

As already explained above, the insert may comprise at least one fixingelement, by means of which the second bearing part may be fixed to thecoupling element. The coupling element may comprise a strap; howevercoupling elements in the form of pins, cords or wires are also feasible.In addition, the two bearing parts may be coupled together by more thanone coupling element. For example two or more parallel straps or pinsmay be provided.

According to a further aspect of the present invention, an implant foruse for adjacently arranged bone plates has a first bearing part, asecond bearing part and a coupling element extending through the secondbearing part for tension-proof coupling of the first bearing part withthe second bearing part, the coupling element being insertable into agap between the bone plates in such a way that the two bone plates arearranged between the two bearing parts, and at least one of the twobearing parts being curved concavely towards the opposing bearing part.The at least one concavely curved bearing part comprises a plurality ofribs, which extend radially outwards from a central portion of thisbearing part.

The individual ribs preferably begin roughly in the middle of thebearing part and may extend at least as far as into the vicinity of theouter circumference of the respective bearing part. The ribs do not allhave to be of the same shape and length. Thus, some ribs may be shorterand other ribs longer.

According to one option between four and ten ribs are provided. The ribsmay be uniformly or indeed non-uniformly spaced from one another. Ineach case two mutually adjacent ribs may in each case define a bearingpart segment. If four or more such segments are provided, each segmentwill exhibit curvature substantially only in the radial direction. Inother words, the curvature of a segment in the circumferential directionwill be markedly less in such a case than the curvature thereof in theradial direction.

According to an embodiment, both bearing parts in each case exhibitconcave curvature and ribs. According to another embodiment, only one ofthe two bearing parts, for example the second, is concavely curved andprovided with ribs.

As already mentioned, various materials and material combinations areavailable for the implant. At least one of the two bearing parts may bemade completely of plastics. It is however also feasible to make onebearing part, consisting of a main body with insert, of a metal/plasticscombination. In such a case, the main body may be made of plastics andthe insert of metal (or vice versa). Bio-absorbable plastics and/orthermoplastics such as polyether ether ketone (PEEK) or the variants andderivatives thereof are examples of possible plastics materials.

The first bearing part and the coupling element may be made in one pieceor indeed as separate components. In the latter case, only afterproduction thereof is the first bearing part connected loosely orrigidly to the coupling element.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention are revealed by thefollowing description of a preferred embodiment. In the Figures:

FIG. 1 is a perspective view of a fully assembled embodiment of animplant;

FIG. 2 is a sectional view of the fully assembled implant according toFIG. 1;

FIG. 3 is a side view of one component of the implant according to FIG.1;

FIG. 4 is a view of the component of FIG. 3 from below;

FIG. 5 is a perspective view of the main body of the upper bearing partof the implant according to FIG. 1;

FIG. 6 is a sectional representation of the main body according to FIG.5;

FIG. 7 is a perspective representation of an insert for fitting into thebearing part main body according to FIG. 5;

FIG. 8 is a first sectional representation of the insert according toFIG. 7; and

FIG. 9 is a second sectional representation of the insert according toFIG. 7.

DETAILED DESCRIPTION

FIG. 1 shows a perspective view of an embodiment of an implant 10 forfixing two adjacently arranged bone plates. The implant is designed tobe used to fix a bone flap to a cranial bone. The implant is alsosuitable however for use as a burr hole cover or as a sternum closure.For the latter use modifications generally known to a person skilled inthe art may be undertaken. In this connection, reference should be madein general to document U.S. Pat. No. 4,802,477.

The implant 10 illustrated in FIG. 1 for fixing a bone flap to a cranialbone is made completely of plastics (for example PEEK). For the implantto be used in particular as a sternum closure, it may be desirable forthe implant to display greater structural stability and therefore to bemade at least in part of metal.

As illustrated in FIG. 1, the implant 10 comprises a first, lowerbearing part 12 and a second, upper bearing part 14, which are coupledtogether in a tension-proof manner by means of a coupling element 16 inthe form of a strap that is flexible with regard to bending movements.Tension is here understood to mean movement of the two bearing parts 12,14 away from one another. As is generally known, to fix the bone flap tothe cranial bone in a first step the strap 16 is arranged in a gapbetween the bone flap and the cranial bone. In a second step the upperbearing part 14 is then displaced either by hand or by means of anapplication tool along the strap 16 towards the lower bearing part 12until the bone flap and the cranial bone are clamped securely betweenthe two bearing parts 12, 14. As a rule between two and six suchimplants 10 are used, which are spaced uniformly from one another aroundthe outer circumference of the bone flap and are optionally arranged inthe region of burr holes.

Preferably, the lower bearing part 12 and the strap 16 form a one-piecefirst component, produced by injection moulding. The gate mark on thestrap 16 is identified by reference numeral 22. The upper bearing part14 threaded onto the strap 16 forms a second component consisting of abearing part main body 18 and an insert 20 fastened in the main body 18.

FIG. 2 shows a sectional representation of the lower part of the implant10 in a plane perpendicular to the long side of the strap 16. FIG. 2clearly shows the curved basic shape of the two bearing parts 12, 14.Each one of the two bearing parts 12, 14 is curved concavely towards theopposing bearing part 14, 12. The rigidity or deformability of thematerial (or the corresponding mixture of materials) used in practicefor the bearing parts 12, 14 may be selected such that, in the course offixing the implant 10 to the bone parts, the central portions of thebearing parts 12, 14 deform resiliently towards one another.

The side of the bearing part 12 facing the bearing part 14 and/or thesurface of the bearing part 14 facing the bearing part 12 may beroughened. The peak-to-valley height may vary in the range between oneand 100 μm. A peak-to-valley height of between five and 30 μm ispreferred, for example an eroded structure according to VDI 3400 no. 45(corresponding to a peak-to-valley height of 18 μm). Such rougheningprevents the implant from slipping during the fixing process and alsoonce implanted.

FIG. 3 shows a side view of the one-piece component consisting of lowerbearing part 12 and strap 16. It is easy to see that surface contouringof the strap 16 in the form of teeth 24 extends from the lower bearingpart 12 over less than around a third of the total length of the strap16. The remaining length of the strap 16 is substantially smooth orroughened and may serve as a point of application for an applicationtool.

Such application tools are generally known and are used to displace theupper bearing part 14 towards the lower bearing part 12. Someapplication tools are furthermore designed to cut off a portion of thestrap 16 which projects above the upper bearing part 14.

The strap 16 is of a first thickness in the region of its smooth orroughened portion which increases gradually in a transitional region 26to a second, somewhat greater thickness in the region of the teeth 24.This material thickness profile simplifies threading of the upperbearing part 14 onto the strap 16.

FIG. 4 shows the component of FIG. 2 (and in particular the lowerbearing part 12) from below. The roughly circular outer contour of thelower bearing part 12 is apparent. The lower bearing part 12 has acentral, circular portion 28 of maximum material thickness, which servesas an anchoring region for the strap 16 (which extends into the plane ofthe drawing in FIG. 3).

A total of six ribs 30 are provided starting from the central portion28. The ribs 30 extend radially from the central portion 28 to the outercircumference of the lower bearing part 12. The ribs 30 have areinforcing function with regard to the structural stability of thelower bearing part 12 for when the strap 16 is exposed to a tensileforce and the bearing part 12 consequently moves into a bearing positionwith the bone plates to be coupled.

Due to the concave basic shape of the lower bearing part 12, each of theribs 30 has a likewise concave curvature along its radial extent. Twoadjacent ribs 30 in each case define a segment 32 with a curvature inthe radial direction approaching the curvature of the ribs 30. Thecurvature of each segment 32 is on the other hand markedly less in thecircumferential direction (i.e. perpendicular to the radial direction).

FIG. 5 is a perspective view of the main body 18 of the upper bearingpart. The shaping of the main body 18 of the upper bearing part 14including the provision of a total of six ribs 34 with segments 36arranged therebetween corresponds to the basic shape of the lowerbearing part 12 explained above in relation to FIG. 4. For this reasonthe features of the main body 18 will not here be looked at in anygreater detail.

FIG. 5 clearly reveals a recess 38 formed centrally in the main body 18for accommodating the insert 20 shown in FIGS. 1 and 2. The insert 20itself is not shown in FIG. 5.

The recess 38 leads into a rectangular opening 40 for accommodating thestrap 16 and connecting elements provided on the insert. FIG. 5 showsthat two sunken counter supports 42, 44 are provided in the recess 38,the function of which will be explained in greater detail below.

FIG. 6 shows a section through the main body 18 in a plane extendingparallel to the longitudinal side of the opening 40 and containing theopening 40. The figure again clearly shows the concave shape of the mainbody 18 and the sunken counter support 44 provided within the recess 38.

FIG. 7 shows a perspective view of the insert 20. The insert 20 has amain body 50, the outer contour of which corresponds to the innercontour of the recess 38 in the main body 18 according to FIG. 5. Theinsert 20 may therefore be accommodated in interlocking andnon-rotatable manner in the recess 38.

Two opposing fixing elements in the form of deflectable tongues 52, 54are provided on the main body 50. Each of the two tongues 52, 54 isresiliently deflectable upwards and downwards in a plane extendingparallel to the strap 16. The mutually facing front ends of the tongues52, 54 are at a distance from one another which is selected as afunction of the thickness of the toothed strap portion such that thefront ends of the tongues 52, 54, as shown in FIG. 2, may cooperate withthe teeth 24 provided on the strap 16.

To achieve an interlocking connection between the tongues 52, 54 and theteeth 24 of the strap 16, the front ends of the tongues 52, 54, asapparent in the sectional representation according to FIG. 8, taper to apoint. This shaping of the front ends of the tongues 52, 54 correspondsto the sawtooth profiling of the teeth 24. The front ends of the twotongues 52, 54 may therefore be accommodated interlockingly between twoadjacent teeth 24. Due to the asymmetric profiling of each of the teeth24, interaction between the tongues 52, 54 of the insert 20 and theteeth 24 of the strap 16 proceeds such that there is less resistance todisplacement of the upper bearing part 14 towards the lower bearing part12 than to displacement of the upper bearing part 14 in the oppositedirection.

In order further to increase the resistance which the interactionbetween the tongues 52, 54 and the teeth 24 provides to movement of theupper bearing part 14 away from the lower bearing part 12 (and thus themaximum achievable clamping force), the counter supports 42, 44explained above in relation to FIG. 5 and provided on the main body 18of the upper bearing part 14 come into play. As shown in FIG. 2, in theinitial state the two tongues 52, 54 rest flat on the two countersupports 42, 44. The counter supports 42, 44 therefore support thetongues in the event of movement of the upper bearing part 14 away fromthe lower bearing part 12. This support opposes downward deflection ofthe tongues 52, 54 and the associated upward mobility of the upperbearing part 14 in FIG. 2, since the front tips of the tongues 52, 54cannot be released from the teeth 24 (or only if considerable force isexpended). The supporting action of the counter supports 42, 44increases markedly overall the maximum clamping force achievable withthe implant 10 over conventional implants and in particular that ofplastics-based implants.

The support of the tongues 52, 54 on the counter supports 42, 44 in theevent of the introduction of a force which urges the two bearing parts12, 14 away from one another naturally greatly increases thecounterforces introduced into the insert 20. These counterforces aredirected towards detaching the insert 20 from the main body 18. For thisreason, efficient anchoring of the insert 20 in the main body 18 isindispensable.

As is shown in FIG. 7, the main body 50 of the insert 20 comprises twoconnecting elements formed as latching elements 56, 58 for fastening theinsert 20 to the main body 18 of the upper bearing part 14. The twolatching elements 56, 58 are of hook-shaped construction and provided onopposing sides of the main body 50 of the insert 20 (sectionalrepresentation according to FIG. 9).

The insert 20 is then inserted into the main body 18 of the upperbearing part 14 in such a way that the latching elements 56, 58 engagebehind the main body 18 of the upper bearing part 14 in the region ofthe short sides of the opening 40. With reference to the sectionalrepresentation of the main body 18 according to FIG. 6, the hook shapesof the latching elements 56, 58 thus engage behind the main body 18 inthe region of steps of the main body 18 provided with reference numerals60 and 62.

The clamping forces which have to be applied to fix a bone plate to acranial bone may then become so significant that the resultantcounterforces bring with them the risk of unlatching of the latchingelements 56, 58. To counter this risk, the latching elements 56, 58 aresupported against the strap 16 with regard to movement into a releaseposition (i.e. in FIG. 9 towards the tongue 54). To this end the mainbody 50 of the insert 20 shown in FIG. 7 comprises guide channels 64, 66in regions adjoining the backs of the latching elements 56, 58 forguiding the strap 16 along its short sides. This situation is alsoillustrated in FIG. 1.

As a result the latching elements 56, 58 are supported against the shortsides of the strap 16 in the event of movement in the direction of arelease position (i.e. in FIG. 9 towards the tongue 54). This supportopposes further movement of the latching elements 56, 58 towards theirrelease position, such that the insert 20 remains reliably anchored inthe main body 18 of the upper bearing part 14. The consequent advantageof achieving extremely high clamping forces is reliably underpinned bythe above-described shaping of the two bearing parts 12, 14 withreinforcing ribs 30, 34.

Although the reinforcing ribs 30, 34, the counter supports 42, 44 andthe support of the latching elements 56, 58 in combination bring aboutparticularly advantageous effects, these technical features may also beused mutually independently, in order to achieve individually thetechnical effects associated with the respective feature. It shouldadditionally be pointed out that the technical features have aparticularly advantageous effect when combined with plastics material,but the implant may also be made partially or completely of metal whileexploiting the same technical effects.

The invention claimed is:
 1. An implant for use for adjacently arrangedbone plates, having: a first bearing part; a second bearing part, whichcomprises a main body and an insert fastened in a recess of the mainbody such that an outer surface of the insert lies flush with an outersurface of the main body, wherein at least one deflectable fixingelement is provided on the insert with an elongated body extendingbetween a first end attached to the insert and a second end having atip; and a coupling element extending through the second bearing partand cooperating with the fixing element for tension-proof coupling ofthe first bearing part with the second bearing part, the couplingelement being insertable into a gap between the bone plates in such away that the two bone plates are arranged between the two bearing parts,wherein a counter support for the fixing element is provided on the mainbody to limit deflection of the fixing element in the event of movementof the second bearing part away from the first bearing part, the countersupport having an abutment surface that continuously supports theelongated body of the deflectable fixing element.
 2. The implantaccording to claim 1, wherein the counter support limits deflection ofthe fixing element towards the first bearing part to a predetermineddistance.
 3. The implant according to claim 1, wherein the secondbearing part has at least two fixing elements opposite one another withregard to the coupling element, and wherein a counter support isprovided for each of the fixing elements.
 4. The implant according toclaim 1, wherein the at least one fixing element takes the form of atongue or claw.
 5. The implant according to claim 1, wherein thecoupling element comprises surface contouring and the at least onefixing element cooperates with the surface contouring for tension-prooffixing of the coupling element to the second bearing part.
 6. Theimplant according to claim 5, wherein the surface contouring takes theform of a series of teeth.
 7. The implant according to claim 6, whereinthe teeth have an asymmetrical profile, such that there is lessresistance to movement of the second bearing part towards the firstbearing part than to movement in the opposite direction.
 8. An implantfor use for adjacently arranged bone plates, having: a first bearingpart; a second bearing part with a main body and an insert fastened in arecess of the main body such that an outer surface of the insert liesflush within the main body, wherein the insert comprises an opening withat least one channel; and a coupling element with at least one keyextending through the opening in the insert for tension-proof couplingof the first bearing part with the second bearing part, the couplingelement being insertable into a gap between the bone plates in such away that the two bone plates are arranged between the two bearing parts,wherein the insert is latched to the main body by at least one latchingelement having an interior surface supported against the couplingelement to oppose movement of the latching element into a releaseposition, and wherein the at least one key of the coupling element isadjacent the interior surface of the at least one latching element whenthe coupling element is inserted into the opening.
 9. The implantaccording to claim 8, wherein the at least one connecting element isprovided on the main body.
 10. The implant according to claim 8, whereinthe at least one connecting element is provided on the insert.
 11. Theimplant according to claim 10, wherein the insert is attached to themain body by means of two connecting elements, which are supportedagainst opposing sides of the coupling element with regard to movementinto a release position.
 12. The implant according to claim 8, whereinthe insert bears at least one fixing element by means of which thesecond bearing part may be fixed to the coupling element.
 13. An implantfor use for adjacently arranged bone plates, having: a first bearingpart; a second bearing part with a recess; an insert with an opening,the insert being adapted to interlock within the recess of the secondbearing part in a non-rotatable manner by at least one latching element;and a coupling element extending through the second bearing part and theopening in the insert for tension-proof coupling of the first bearingpart with the second bearing part, the coupling element being insertableinto a gap between the bone plates in such a way that the two boneplates are arranged between the two bearing parts, wherein each of thetwo bearing parts being curved concavely towards the opposing bearingpart, and wherein each of the first and second concavely curved bearingpart has an outer surface with a plurality of ribs extending radiallyoutwards from a central portion of the bearing part.
 14. The implantaccording to claim 13, wherein between four and ten ribs are provided.15. The implant according to claim 13, wherein two adjacent ribs in eachcase define a bearing part segment and each segment exhibits curvaturein the radial direction.
 16. The implant according to claim 13, whereinat least one of the two bearing parts is made of plastics.
 17. Theimplant according to claim 16, wherein the implant is made at least fromPEEK.
 18. The implant according to claim 13, wherein the first bearingpart is made in one piece with the coupling element.
 19. The implantaccording to claim 18, wherein the first bearing part is roughened onits surface facing the second bearing part and/or the second bearingpart is roughened on its surface facing the first bearing part.
 20. Theimplant according to claim 13, wherein the insert is fastened to thesecond bearing part adjacent the opening so as to lie flush within thesecond bearing part, and wherein at least one deflectable fixing elementis provided on the insert.